1. Field of the Invention
The present disclosure relates to a highly dense and nano-grained NTC thermistor thick film and a method for preparing the same.
2. Description of the Related Art
In general, the term “sensor” refers to a device which can respond to external stimuli or changes in the environment to enable appropriate reciprocal measures to be taken. Sensors may be varied and include, for example, temperature, pressure, gas, and infrared sensors. As the ranges of use for sensors become broader in various industries, the principles, kinds, and requirements of sensors are becoming more diverse and important.
A thermistor is a temperature sensor that changes in resistance according to a change in temperature. Thermistors include Negative Temperature Coefficient (NTC) thermistors and Positive Temperature Coefficient (PTC) thermistors. These are typical examples of electrically conductive ceramics.
An NTC thermistor is one that operates on the principle of its resistance decreasing as the temperature increases. Most thermistors fall into the NTC category, which has the characteristic semiconductor property of resistance exponentially decreasing over a wide temperature range.
A PTC thermistor is a special thermistor that operates on the principle of its resistance drastically increasing when the temperature surpasses a critical level, which is due to changes in dielectric characteristics that affect electrical properties which may bring about a big change in resistance even in a very narrow temperature range in regions between particles.
Referring to the related art of thermistors, studies had been conducted on materials and compositions of thermistors in the UK and USA from the late 1930s to the early 1940s, and oxides of the transition metals Mn, Ni, Co, Fe, Cu, etc. were used as raw materials to develop composite oxide products made of two or more oxides. In addition, a Mn—Ni oxide-based composite sintered body was developed at Bell Laboratories in the US in 1946, named “thermistor”, and commercialized. Then, at the turn of the 1950s, the thermistor began to draw attention as a temperature sensor due to development in the tricomponent system of Mn—Co—Ni oxides and later materials containing Fe—Cu oxides and rapid advancement in manufacturing technology.
The thermistor may be classified into disc, diode, chip (epoxy and glass) types using conventional ceramic manufacturing technology; surface-mounted types using thick film or thick film stacking processes; and thin film types. Because the thermistor is inexpensive and has a high rate of resistance variation, it is easy to manufacture as a sensor by which temperature may be precisely controlled and managed. Relatively high resistance values may also be obtained at room temperature.
Because thermistor materials having an NiMn2O4-based spinel grain structure, which are widely used in Negative Temperature Coefficient (NTC) thermistors, are required in the application of film-type thermistors including thick and thin film type thermistors, methods of forming thick films using a screen printing method for sintering are commonly used. The method is a low-cost, stabilized process for industrial use and suitable for mass-production. However, materials have poor sintering properties and must be subjected to a heat treatment (sintering process) at high temperatures. Because the materials also contain a large amount of organic additives during screen printing, the sintering density of the materials after sintering is not as high as expected. Due to a high temperature (900° C. or higher) sintering which is an essential process of screen printing, there is a limitation in available substrates. In the case of a substrate such as glass or polymer which is modified or melted at high temperatures, or a substrate using materials which easily inter-diffuse into NTC compositions at high temperature, it is impossible to prepare an NTC thermistor film with a preparation method using screen printing.
Although several attempts to prepare highly dense thermistor thin and thick films using electron-beam evaporation, pulsed laser deposition, RF reactive sputtering, etc. have been made in order to overcome these problems, these methods require high-vacuum devices and have many difficulties in commercialization due to low deposition rates in the range of a few nanometers per minute.
Thus, the present inventors have studied methods for preparing highly dense thick film, by which thick films may be deposited at room temperature without high temperature sintering processes, confirmed that a room temperature powder spray in vacuum (so called Aerosol-Deposition; AD) may be used for rapid deposition, and that highly dense and nano-grained thick films may be prepared without additional heat treatment processes, and completed the present invention.